1. Field of the Invention
The present invention relates to methods for producing semiconductor lasers, semiconductor lasers, optical pickups, and optical disk drives. More specifically, the present invention is suitable for application to, for example, ridge-stripe semiconductor lasers with an edge window structure using nitride-based III-V compound semiconductors and optical pickups and optical disk drives using such semiconductor lasers as light sources.
2. Description of the Related Art
To increase the maximum optical output of a semiconductor laser, it is effective to introduce an edge window structure where a window transparent to light emitted from an active layer is provided at an edge of a resonator.
For GaInP-based red light-emitting semiconductor lasers, it is effective to form an edge window structure by diffusing zinc atoms into a semiconductor layer forming a laser structure near a position corresponding to an edge of a resonator after the growth of the semiconductor layer (see, for example, Japanese Unexamined Patent Application Publication No. 2005-45009 (Patent Document 1)). In this case, the edge window structure is formed by diffusing zinc atoms into the semiconductor layer near the position corresponding to the edge of the resonator so that the bandgap energy thereof can be locally increased.
Recent high-density optical disk drives include semiconductor lasers using nitride-based III-V compound semiconductors as light sources. Most nitride-based III-V compound semiconductors are more thermally and mechanically stable than GaInP-based semiconductors. Consequently, it is difficult to form an edge window structure in a semiconductor laser using a nitride-based III-V compound semiconductor by diffusion of foreign atoms and wet etching, which are effective for GaInP-based red light-emitting semiconductor lasers.
Accordingly, various proposals and experiments have so far been made as to methods for forming an edge window structure in a semiconductor laser using a nitride-based III-V compound semiconductor. Methods proposed so far for forming an edge window structure will be described below.
According to some proposed methods, after a laser bar is formed by cleavage, an edge window structure is formed by increasing the bandgap energy near an edge of a resonator by removing indium through exposure to hydrogen plasma or laser irradiation (see, for example, Japanese Unexamined Patent Application Publication Nos. 2006-147814 (Patent Document 2) and 2006-147815 (Patent Document 3)). These methods, however, result in significant capital investment because they use a high-vacuum chamber apparatus, and are also generally disadvantageous in terms of productivity because the edge of the resonator is processed after the cleavage.
Many proposals have been made as to the following method (see, for example, Japanese Unexamined Patent Application Publication Nos. 2004-134555 (Patent Document 4) and 2003-60298 (Patent Document 5), PCT International Publication No. WO 03/036771 (Patent Document 6), and Japanese Unexamined Patent Application Publication No. 2002-204036 (Patent Document 7)). Specifically, first, a semiconductor layer forming a laser structure is epitaxially grown on a substrate. A portion of the semiconductor layer corresponding to an edge of a resonator is then etched by reactive ion etching (RIE). Subsequently, a nitride-based III-V compound semiconductor layer with a large bandgap energy is epitaxially grown on the etched portion. In this method, however, light absorption and local heat generation may occur during laser operation because a surface level appears at the surface etched by RIE.
Another example is a method of forming an edge window structure by epitaxially growing a semiconductor layer forming a laser structure on a substrate having geometrical steps formed by RIE or insulating film deposition (see, for example, Japanese Unexamined Patent Application Publication Nos. 2005-191588 (Patent Document 8), 2005-294394 (Patent Document 9), 2003-198057 (Patent Document 10), and 2000-196188 (Patent Document 11)). This method is intended to use a cladding layer with a larger bandgap energy than an active layer as an edge window structure in a direction in which laser light travels. A typical example is shown in FIG. 25. In FIG. 25, a recess 101a is formed in a main surface of a substrate 101 by RIE patterning. An n-type semiconductor layer 102, an active layer 103, and a p-type semiconductor layer 104 are sequentially grown on the substrate 101, and a p-side electrode 105, an isolation electrode 106, and a pad electrode 107 are formed on the p-type semiconductor layer 104. This method, however, has the following problem. The recess 101a in the substrate 101 forms steep geometrical steps in the n-type semiconductor layer 102, the active layer 103, and the p-type semiconductor layer 104, thus causing waveguide loss near the steps. In addition, this structure may not function as an effective edge window structure because it is not intended to form a transparent region by widening the bandgap of the active layer 103 near the edge of the resonator.
According to another example of the related art, semiconductor lasers are produced using a nitride-based III-V compound semiconductor substrate including a first region formed of a single crystal and having a first average dislocation density and second regions arranged in the first region and having a second average dislocation density higher than the first average dislocation density (see, for example, Japanese Unexamined Patent Application Publication No. 2003-124572 (Patent Document 12)). An example of such a nitride-based III-V compound semiconductor substrate is one including a first region formed of a single crystal and having a first average dislocation density and second regions having a second average dislocation density higher than the first average dislocation density and periodically arranged in parallel in the first region so as to extend linearly. When a nitride-based III-V compound semiconductor layer forming a laser structure is grown on the nitride-based III-V compound semiconductor substrate, the second regions of the nitride-based III-V compound semiconductor substrate are transcribed in the nitride-based III-V compound semiconductor layer. Laser chip regions are defined on the nitride-based III-V compound semiconductor substrate so that the second regions are not included in laser stripes.